A long wire is wrapped around an object and

connect to a battery. The direct current

supplied in the wire produces magnetic field

that magnetizes the object.

(i) Inserting a soft iron core in the solenoid

(ii) Passing large current through the wire

(iii) Increasing the number of turns of the

wire

The combined effect of all turns of wire

produces a field similar to the bar magnet. A

coil of several turns is called a solenoid

Polarity of the solenoid

(i) Polarity of a magnet can be determined

by the direction of the current “the clock

rule”

“When looking at the object, end that the

current is flowing in a clockwise direction is

the S – pole”

Solenoid

If it flows in anticlockwise direction, the end

acts as the N – pole

A solenoid is an electrical device consisting

of a coil wire of several turns along which a

magnetic field is formed when electric

current is passed through it

(ii) Also, the right-hand grip rule (the

solenoid rule) can be used to show the

polarity of the magnetised material.

Factors affecting the strength of magnetic

field in the solenoid

(i) Amount of current

Using large current increases the strength of

magnetic field produced

Right hand grip rule state that “If the

fingers of the right-hand grip the solenoid in

the direction of current, then the thumb

points in the N – pole of the solenoid”

(ii) Number of turns

The greater the number of turns produces

larger magnetic field

(iii) Use of soft iron material inside the

solenoid

Inserting soft iron material increases the

magnetic field strength of the solenoid

This rule is also known as the solenoid rule

Note:

Ways to increase the field strength of the

magnet by the solenoid industrially

(i)

To make a permanent magnet,

steel must be used

(ii)

The current is switched on for a

short time only, because

overheating can damage the coil

The material does not become

strongly magnetised if the current

is left on for a long time

Magnetization can also be done

by induction and hammering

methods

(iii)

(iv)

INDUCTION

Magnetic induction is process where a

magnetic material gets magnetised due to

being close to a magnet

It was observed that the razor blade on the

wooden block was attracted to the magnet

while the other on the iron block was not.

Explain.

Answers

In (a) the magnet induces magnetism into

the razor blade which in turn induces the

iron block to acquire opposite polarity. This

causes the razor blade to remain attracted to

the iron block.

From the diagram, the magnet induces

opposite poles to the pins, hence the pins

are attracted to the magnet

In (b) no magnetism is induced into the

wooden block and so the razor blade is

attracted

Example 09

The tips of the pins have similar pole (S

– pole) hence repel from each other.

Two steel pins were attracted by a magnet as

shown below

Note that: the tips of the pins are far

apart, because they repel from each other

due to the presence of induced like poles

at the tips

Example 08

Two similar razor blades were placed one on

a wooden block and the other on an iron

block as shown in the figure below.

(a) Give a reason why the ends of the

two pins move apart instead of being

parallel

(b) What observation would be made if a

south pole of another magnet is brought

between the two pins

. If a permanent magnet is heated or vibrated

in the absence of magnetic field, the

magnetization can be destroyed

Answers

(ii) Random stroking

(a) The pins repel because they acquire

like magnetic poles at their free ends

(b) The pins move further apart due to

repulsion from the south pole

Random stroking of one magnet with

another can demagnetize the magnet being

stroked

(iii) Using alternating current

Wrapping a wire coil around the magnet and

connecting the coil to a source of alternating

current will eliminate the magnetic

alignment

HAMMERING METHOD

In this method, one end of a steel bar lying

in the N – S direction of the earth is struck

with the harmer several times. The steel

becomes a weak magnet

(iv) Repeated hammering

Repeated hammering or dropping down the

magnet can also distort the dipole alignment

Note:

(i) When the magnet is placed inside the

solenoid with its axis pointing in an East –

west direction and the current is switched on

for few seconds and then withdrawn from

the solenoid in the east – west direction with

alternating current still flowing in the

solenoid, the magnet loses its magnetism

The lower end of the steel bar (the end

which faces the direction of the Earth’s

magnetic field) becomes N – pole

Demagnetization

Demagnetization is the process of

destroying the magnetic property of a

material

This process involves the destruction of the

dipole alignment in the magnetic material.

(ii) The magnet is placed in an east – west

direction, so that it is not left with any

magnetism due to the induction in the

earth’s magnetic field

Methods of demagnetization

The magnetism of the material is lost

(destroyed) through the following ways.

(iii) When this bar is brought near some iron

fillings, it does not attract them. This proves

that a bar has been demagnetized.

(i) Heating in the absence of magnetic field

Example 01

Study carefully the diagram below then

answer the next questions

Answers

Because ferromagnetic material greatly

increases the strength of magnetic field

produced by the current, since is easily

magnetised

Storage of magnets

In order to maintain the magnetism for a

long period of time, the following practices

have to be observed

(i)

Avoid storing magnets in places

where they may come into

contact with ferrous objects such

as steel shelves and tools

(a) Why is the solenoid and bar magnet

placed in an East – west direction?

Answers

(ii)

Store magnets in pairs with the

unlike poles facing each other

with magnetic keepers

This prevents the Earth’s field from

inducing or retaining magnetism in the bar

during experiment

Note: pieces of magnetic keepers are

used at the end to preserve the strength

of the magnets

(b) What is observed when the bar magnet is

brought near the iron fillings?

A magnetic keeper is a ferromagnetic

bar which is placed across the poles of a

permanent magnet to preserve its

strength

Answers

The bar magnet does not attract the iron

fillings. This shows that the bar magnet has

been demagnetized.

Example 02

Mention three ways that a magnet can lose

its magnetism

Answers

(i)

(ii)

Random stroking

It preserves the strength of the magnet

completing the magnetic loop

Hammering, heating or vibration

in absence of external field

Electric method by using

alternating current

(iii)

Do not overheat the magnets

This may cause harmful

structural changes in the magnet

Example 03

(iv)

Do not store or keep magnets

near strong magnetic or electric

fields

Why is the core of an electromagnet made of

ferromagnetic material?